1. Field of the Invention
The present Invention relates to an air scavenging two-stroke cycle engine used as a driving source of a small machine such as a brush cutter.
2. Description of the Related Art
The conventional air scavenging two-stroke cycle engine is shown in FIG. 6 (see Japanese Patent Application Publication No. Hei. 10-121973) FIG. 6 shows a state in which a piston 57 is positioned at a top dead center. This engine is provided with a scavenging block 53 between a cylinder 51 and a crank case 52, in which a plurality of scavenging passages 54 are formed. In a state in which the piston 57 is positioned in the vicinity of a bottom dead center, the plurality of scavenging passages 54 make a combustion chamber 50 in the cylinder 51 and a cylinder head 61 communicate with a crank chamber 52a in the crank case 52. These scavenging passages 54 are connected to air passages 55 for introducing air from a portion of an intake passage(not shown) that is situated downstream from an air cleaner. Check valves 56 comprising reed valves for opening or closing openings 55a of the air passages 55 that are opened to the scavenging passages 54 are provided on inner surfaces of the scavenging passages 54.
In the engine so configured, a piston 57 in the cylinder 51 moves upward from a bottom dead center, which sequentially closes a scavenging port 54a of the scavenging passage 54 that is opened in the combustion chamber 50 and an exhaust port (not shown) to thereby cause an internal pressure of the combustion chamber 50 to be increased and an inside of the crank chamber 52a and the scavenging passage 54 to have negative pressures. Thereby, the intake passage (not shown) connected to the crank chamber 52a is opened and fuel-air mixture is introduced into the crank chamber 52a. Simultaneously, the check valve 56 is opened and air from the air passage 55 is Introduced into the scavenging passage 54.
Just before a top dead center of the piston 57, the fuel-air mixture in the combustion chamber 50 is ignited by an ignition plug 58 and explodes, and then the piston 57 begins to move downward. Along with this downward movement of the piston 57, the exhaust port is first opened and combustion gas in the combustion chamber 50 is then discharged externally to thereby cause the internal pressure of the combustion chamber 50 to be reduced. Then, the scavenging port 54a of the scavenging passage 54 is opened. Prior to the fuel-air mixture, the air introduced into the scavenging passage 54 is ejected into the combustion chamber 50 where the internal pressure has been reduced and expels the residual combustion gas inside thereof externally from the exhaust port, to thereby perform initial scavenging operation in the combustion chamber 50 by using the air. In this case, the scavenging air blowing by from the exhaust port is air, and therefore blow-by of the fuel-air mixture hardly occurs. Following this, the fuel-air mixture in the crank chamber 52a is introduced through the scavenging passage 54 into the combustion chamber 50, whereby scavenging operation is completed. Thereafter, the above cycle is repeated.
However, in the engine so configured, since the check valve 56 is provided in the scavenging passage 54, there is a possibility that the scavenging air or the fuel-air mixture does not smoothly flow through the scavenging passage 54 into the combustion chamber 50 due to an obstacle such as the check valve 56. In addition, it is necessary to provide the check valves 56 as many as the scavenging passages 54. Accordingly, the man-hour of assemblies is increased with an increase in the number of the check valves 56 or stoppers thereof and fitting screws, leading to high cost. Further, since the scavenging block 53 is formed separately from the cylinder 51 to allow the check valve 56 to be provided in the scavenging passage 54, the number of parts such as screws or gaskets with which the scavenging block 53 is mounted is increased, also leading to high cost.
In the above configuration, the initial scavenging operation is not necessarily performed in the combustion chamber 50 only by using air. More specifically, although the residual fuel-air mixture in the scavenging passage 54 after a previous scavenging stroke is returned to the crank chamber 52a due to the air introduced through the air passage 55, the fuel-air mixture and the air tend to be mixed in the scavenging passage 54 because of small length and large cross-sectional area of the scavenging passage 54. In addition when the air is introduced into the scavenging passage 54, the fuel-air mixture remains in an end portion of the scavenging passage 54, i.e., between the scavenging port 54a and the opening 55a of the air passage 55. For this reason, the initial scavenging operation is performed in the combustion chamber 50 by using the air including the fuel-air mixture, and the fuel-air mixture blows by from the exhaust port.
Another example of the air scavenging two-stroke cycle engine is described in Japanese Patent Application Publication No. Hel. 9-268918. In this engine, an air passage is connected to a scavenging passage through which the fuel-air mixture is introduce into the combustion chamber, and the air is ejected into the combustion chamber from the scavenging port. In this case, it is also necessary to provide the check valves as many as the scavenging passages at connected portions of the scavenging passages and the air passages. Therefore, the number of parts is increased. In addition, the air and the fuel-air mixture are mixed in the scavenging passages, and therefore the initial scavenging operation cannot be performed only by using air.
In consideration of such circumstances, it is an object of the present invention to provide an air scavenging two-stroke cycle engine which is capable of smoothly supplying fuel-air mixture from a crank case into a combustion chamber, reducing the number of parts to thereby reduce the man-hour of assemblies and cost, and performing scavenging operation only by using air in the initial stage of the scavenging stroke to suppress blow-by of the fuel-air mixture from an exhaust port, in order to realize high fuel efficiency and reduction of concentration of HC emission.
In an aspect of the present invention, an air scavenging two-stroke cycle engine that introduces fuel-air mixture introduced into a crank case provided on a lower portion of a cylinder through an intake port into a combustion chamber in an upper portion of the cylinder from a first scavenging port and discharges a combustion gas externally from a discharge port, by up and down movement of a piston in the cylinder, comprises: an air passage; a communicating passage; and a second scavenging port, wherein the second scavenging port is formed in the cylinder such that it is disposed above the intake port and has an upper end higher than an upper end of the first scavenging port and lower than an upper end of the discharge port, the air passage is connected to the second scavenging port for introducing air through a check valve, and a portion of the air passage that is situated downstream from the check valve communicates with an inside of the crank case through a communicating passage.
In the engine so configured, in a scavenging stroke, scavenging operation is firstly performed by using air such that the second scavenging port is opened, scavenging air filled in the communicating passage is ejected into the combustion chamber, and then combustion gas in the combustion chamber is expelled externally from the exhaust port. Following this, the first scavenging port is opened and fuel-air mixture in the crank case is thereby introduced into the combustion chamber, whereby scavenging operation is completed.
When the scavenging operation performed in the combustion chamber by using the air is completed, the fuel-air mixture is tilled in a region of the communicating passage, corresponding to a region from the crank case to the vicinity of the second scavenging port. A part of this filled fuel-air mixture is pushed back into the crank case by air introduced into the communicating passage from the air passage when the check valve is opened in an intake stroke, and the residual remains in the communicating passage closer to the crank case as being separated from the air. At this time, a portion of the fuel-air mixture which has not been pushed back by the air might remain in the air passage in vicinity of the second scavenging port. However, if a portion where the communicating passage is connected to the air passage is formed closer to the second scavenging port, then the fuel-air mixture in the vicinity of the second scavenging port is sucked into the communicating passage by suction force of the air flowing into the communicating passage, and no residual of the fuel-air mixture occurs. Therefore, in an initial stage of scavenging operation, only the air in the communicating passage is ejected into the combustion chamber from the second scavenging port and, by using this air, scavenging operation is performed in the combustion chamber. For this reason, blow-by of the fuel-air mixture is suppressed. As a result, high fuel efficiency is achieved and concentration of HC emission is reduced.
In addition, in this engine, no obstacle such as the check valve is provided in the first scavenging passage connecting the crank case and the combustion chamber, and therefore the fuel-air mixture in the crank case is smoothly supplied from the first scavenging port into the combustion chamber. Further, the number of parts is reduced and cost is correspondingly reduced.
Preferably, the second scavenging port is disposed at a position opposite to the exhaust port in a diameter direction of the cylinder. With this configuration, since the air from the second scavenging port travels transversely across the combustion chamber toward the exhaust port, the combustion gas is quickly scavenged. As a result, scavenging efficiency is improved.
Preferably, the second scavenging port is opened to be directed obliquely upward so that it supplies air along a convexly curved upper surface of the piston. With this configuration, the air smoothly flows along the upper surface of the piston, and therefore the scavenging efficiency of the combustion gas scavenged by using the air from the second scavenging port is further improved.
Preferably, a part of the communicating passage is constituted by a connecting pipe disposed externally of the engine. With this configuration, sufficient length of the communicating passage is ensured for a small engine. Therefore, the air introduced into the communicating passage is prevented from going into the crank case and being mixed with the fuel-air mixture. Further, the communicating passage is made narrower while keeping the volume of the air required for scavenging operation. In other words, since the passage area (a cross-sectional area of the passage) is made smaller, it hardly occurs that the air and the fuel- air mixture pushed back to the crank case side by the air are mixed in the communicating passage, that is, they are separated from each other satisfactorily. Accordingly, in the initial stage of the scavenging operation, only the air is ejected from the second scavenging port through the communicating passage for scavenging operation in the combustion chamber.
This object, as well as other objects, features and advantages of the invention will become more apparent to those skilled in the art from the following description taken with reference to the accompanying drawings.